This invention relates to magnetic elements and, in particular, to semi-hard magnetic elements and methods of making same.
As used herein, the term semi-hard magnetic element means a magnetic element having semi-hard magnetic properties which are defined herein as a coercivity in the range of about 10-500 Oersted (Oe) and a remanence, after removal of a DC magnetization field which magnetizes the element substantially to saturation, of about 6-9 kilogauss (kG). Semi-hard magnetic elements having these semi-hard magnetic properties have been used in a number of applications. In one particular application, the elements serve as control elements for the tags in a magnetic electronic article surveillance (EAS) system. A magnetic tag of this type is disclosed, for example, in U.S. Pat. No. 4,510,489.
In the tag of the '489 patent, a semi-hard magnetic element is placed adjacent to a magnetostrictive amorphous element. By magnetizing the semi-hard magnetic element substantially to saturation, the resultant remanence magnetic induction of the magnetic element arms or activates the magnetostrictive element so that it can mechanically resonate or vibrate at a predetermined frequency in response to an interrogating magnetic field.
This mechanical vibration results in the magnetostrictive element generating a magnetic field at the predetermined frequency. The generated field can then be sensed to detect the presence of the tag. By demagnetizing the semi-hard magnetic element, the magnetostrictive element is disarmed or deactivated so that it can no longer mechanically resonate at the predetermined frequency in response to the applied field.
The semi-hard magnetic elements presently used for the above tags are formed from materials commercially sold under the tradenames or trademarks Arnokrome-3, Crovac 10/130 and Vicalloy. These materials are crystalline and contain various amounts of iron, cobalt, chromium, vanadium and possibly other constituents. The materials also have certain disadvantages.
One disadvantage is that the materials are costly. Another disadvantage is that forming the materials involves complicated and lengthy processing which is a significant factor contributing to their high cost.
Generally, this processing involves many steps including multiple rolling, annealing and slitting procedures. Also, a large amount of mechanical work is required to reduce the material thickness into the 50 um range which is the typical range required for the magnetic elements.
The need for lengthy processing not only increases costs, but makes it more difficult to control the magnetic properties of the formed magnetic materials. This leads to magnetic elements and, therefore, tags which may be unstable and therefore subject to failure.
U.S. Pat. No. 4,298,862 discloses another magnetic tag in which magnetic elements are used as part of the tag to control activating and deactivating the tag. In the tag of the '862 patent, a soft amorphous ferromagnetic material is used as the signal generating element. In this case, the amorphous material causes perturbations at harmonics of an interrogating magnetic field. These perturbations are then detected to sense the presence of the tag.
The '862 patent mentions a number of soft amorphous ferromagnetic materials which can be used in the tag of the patent. These materials contain various constituents including, for example, Fe, Co, Si, B and P. Also some of these materials are iron rich, contain at least 50 atomic percent iron. The particular iron rich materials mentioned are Fe-B, Fe-Mo-B and Fe-C-Si-B.
In the tag of '862 patent, the magnetic elements are formed as surface portions of the amorphous ferromagnetic material with the bulk of the material remaining amorphous. By magnetizing these elements, the amorphous material becomes biased so that it can no longer perturb the interrogating signal, thereby deactivating the tag.
The '862 patent teaches forming the magnetic elements by crystallizing spaced surface portions of the amorphous ferromagnetic material as or after the material is formed into a strip or ribbon. Controllably processing the amorphous material to form these surface portions is not easily realizable. Thus, this technique does not offer an entirely satisfactory way of providing magnetic elements for a magnetic tag, nor is it usable for tags where it is desired that the magnetic control elements be separate elements from the signal generating elements.
It is also known from an article entitled "Magnetization Process in Devitrified Glassy Alloy" (R. C. O'Handley, et al., American Institute of Physics, 1965, pp. 3563-65), that magnetic material comprised of amorphous Co.sub.84 Nb.sub.10 B.sub.6 can be made semi-hard by annealing the material to crystallize its bulk. However, this cobalt rich material is relatively expensive.
It is, therefore, an object of the present invention to provide a semi-hard magnetic element which is relatively easy to manufacture and less expensive than those presently in use.
It is a further object of the present invention to provide a semi-hard magnetic element which is stable.
It is also an object of the present invention to provide a method of making a semi-hard magnetic element which meets the above objectives.
It is a further object of the present invention to provide a magnetic tag and a magnetic EAS system utilizing a semi-hard magnetic element which meets the above objectives.